Wind turbines use wind energy typically to generate electricity. A conventional wind turbine includes a rotor mounted on a tower. The rotor may turn up to about 30 rpm to 120 rpm dependent on the wind turbine in a steady wind of about 12 meters per second and connect to an alternator/generator through a speed increasing transmission. Typical generators include synchronous or asynchronous generators and require a constant output shaft speed of about 1200 to 1800 rpm, depending on the type of generator, to produce quality power. Although variable speed generators are available, the power output of a variable speed generator must be conditioned before it can be fed into a power grid.
The aerodynamic efficiency of a wind turbine is very much dependent on the control of the rotor speed. Many wind turbines have a gear train that is connected to the rotor. It is a continuing problem with wind-driven turbines to provide a cost-effective method of smoothing the torque generated by the rotor so as to reduce torque fluctuations in the drive train to the generator. Turbulence is one of the sources of gear box problems, and torque ripple coming into the grid.
Wind turbines using an open loop hydraulic system in place of mechanical transmissions are also known. However, conventional hydraulic pumps require input speed of a minimum of about 300 to 500 rpm to produce usable hydraulic pressure. As a result, a mechanical speed increaser is still required between the rotor and the hydraulic pump. Most hydraulic systems for wind turbines utilize an open loop system. For example, in U.S. Pat. No. 4,503,673, a positive displacement hydraulic pump connected to a variable displacement hydraulic motor is disclosed. In this system, similar to other open loop systems, the hydraulic pump is elevated on the tower but the hydraulic motor, hydraulic fluid reservoir and generator are on the ground. Although it is generally advantageous to reduce the tower load, this arrangement necessitates long hydraulic fluid hoses to and from the hydraulic pump, which is disadvantageous. Additionally, in the system disclosed in U.S. Pat. No. 4,503,673, complex hydraulic controls are used to feather the rotor propeller blades in order to deal with excess pressure in the hydraulic circuit.
In an open-loop hydraulic system, when the rotor is driven at high speed, excess hydraulic pressure may be diverted by “dumping” pressure to maintain a constant generator speed. This energy dissipation generates tremendous amounts of heat and active cooling or heat exchanging is necessary. For example, in U.S. Pat. No. 4,149,092, a hydraulic system for water and wind driven turbines is disclosed which includes a shunt-connected energy dissipator. In response to high pressure caused by high wind and rotor speeds, the displacement of the hydraulic motor decreases, further increasing system pressure. As a result, the hydraulic fluid is diverted into the energy dissipator. The dissipator converts hydraulic energy into heat which is removed by a heat exchanger
In a conventional hydrostatic transmission, a prime mover drives a pump which converts power into hydraulic pressure. The hydraulic pressure is then transmitted to a hydraulic motor which converts the pressure back into power, which may then be used to power a load. The hydraulic fluid returns to a reservoir, which feeds the pump. In the context of prior art wind turbines, the rotor is the prime mover and the load is the electrical generator.
In the field of hydrostatic transmissions, an “overrunning” or “overhauling” load condition is a state where a hydraulic motor is mechanically driven by its load, rather than the converse. An example of an overrunning load is the instance when a vehicle with a hydrostatic transmission is driven down a downgrade. In that case, the road wheels impart torque to the hydraulic motor which in turn acts on the pump. It is assumed that both the motor and the pump are pressure reversible. The pump may then regenerate horsepower back into the prime mover. In effect, the pump and motor exchange functions and energy flows in reverse. This ability of the pump to regenerate power in the prime mover is referred to as dynamic braking capability.
Harvey et al. in U.S. Pat. No. 7,418,820 teach that a closed loop hydraulic system which may effectively deal with an “overrunning” load condition may successfully be applied to a wind turbine to provide efficient transmission of energy from the turbine rotor to the generator. They also teach that a low-speed, high torque hydraulic motor driven by the wind turbine rotor at a low speed to create an overrunning load condition, may efficiently drive such a hydraulic system. The motor acting as a pump is preferably directly driven by the rotor, without any speed increasing gears. Similarly, a variable displacement pump which is driven by the overrunning load to reverse its function may be effectively used to drive the electrical generator. The reversal of component roles permits the electrical startup of the rotor in a startup procedure.
Harvey et al. failed to recognize that the aerodynamic efficiency of a wind turbine is very much dependent on the control of the rotor speed. Hence, they fail to teach or suggest any mechanism for controlling rotor speed. Additionally, they provide a single (variable displacement) hydraulic motor connected to a generator. But, we have found that a single hydraulic motor typically results in unacceptably low efficiencies at the low to mid power range.
Therefore, there is a need in the art for a wind turbine system utilizing a closed loop hydrostatic transmission which mitigates the difficulties of the prior art.